The invention relates to a circuit device for supplying an alternating current of frequency f to a lamp, which circuit device is provided with a DC-AC converter comprising
input terminals for connecting the circuit device to a supply voltage source supplying a DC voltage,
a first branch including a series arrangement of a first switching element and a second switching element,
a control circuit coupled to respective control electrodes of the switching elements for rendering the switching elements conducting and non-conducting,
a load branch shunting one of the switching elements and provided with a series arrangement of an inductive element and terminals for accommodating the lamp.
Such a circuit device is disclosed in EP 0323676. In such a circuit device, the power consumed by the lamp can be adjusted, for example, by adjusting the frequency f of the control signal. A drawback of this way of adjusting the power consumed by the lamp resides in that the connection between the frequency of the control signal and the power consumed by the lamp is not unambiguous throughout the range of power consumed by the lamp. Particularly in the case of a comparatively low power consumption by the lamp, this may give rise to instabilities in the lamp operation. Another possibility of adjusting the power consumed by the lamp is to adjust the periods during which the switching elements are conducting in each period of the control signal, while the frequency of the control signal remains constant. This can be carried out symmetrically, which means that each one of the switching elements is conducting during an equal period of time in each period of the control signal. However, this can also be carried out asymmetrically, which means that the time interval during which the first switching element is conducting is unequal, in each period of the control signal, to the time interval during which the second switching element is conducting. In addition, a distinction can be made between a situation wherein one of the switching elements is conducting at any instant in a period of the control signal (apart from the very short time interval during which the conducting switching element is rendered non-conducting and the non-conducting switching element is rendered conducting), and a situation wherein there are time intervals during which neither switching element is conducting. In practice it has been found that asymmetrically driving the switching elements gives rise, for certain unpredictable values of power consumed by the lamp, to instabilities in the lamp. If the switching elements are symmetrically driven, a reduction of the duration during which each of the switching elements is conducting in a period of the control signal means that, during each period of the control signal, there are time intervals wherein both switching elements are non-conducting. It has been found that this way of driving the switching elements also gives rise to instabilities in the lamp, however, the values of power consumed by the lamp are predictable.
It is an object of the invention to provide a circuit device by means of which the power consumed by the lamp can be adjusted in a comparatively large range without instabilities developing in the lamp.
To achieve this, a circuit device as mentioned in the opening paragraph is characterized in accordance with the invention in that the control circuit generates a control signal at a frequency f during operation of the lamp,
for rendering the first switching element, in each first half period of the control signal, successively conducting, non-conducting and conducting during, respectively, a first, a second and a third time interval, the second switching element always being conducting when the first switching element is non-conducting, and non-conducting when the first switching element is conducting, and
for rendering the second switching element, in each second half period of the control signal, successively conducting, non-conducting and conducting during, respectively, a fourth, a fifth and a sixth time interval, the first switching element always being conducting when the second switching element is non-conducting, and non-conducting when the second switching element is conducting, and
in that the control circuit is further provided with a dimming circuit for setting the duration of the second and the fifth time interval.
During operation of a circuit device in accordance with the invention, the control signal renders the switching elements alternately conducting and non-conducting. During each first half period of the control signal, the current in the load branch and hence also the current through the lamp has an average value measured in a first polarization direction. During each second half period of the control signal, the current in the load branch and hence also the current through the lamp has an average value measured in a second polarization direction. As a result, an AC current of frequency f flows in the load branch. Apart from the very short time interval during which, in succession, the conducting switching element is rendered non-conducting and the non-conducting switching element is rendered conducting, one of the switching elements is conducting at any instant of a period of the control signal. When the duration of the second time interval and the duration of the fifth time interval are both zero, the power consumed by the lamp is maximal and one of the switching elements is continuously conducting during each half period of the control signal. If the dimming circuit sets the duration of the second time interval and the duration of the fifth time interval at a value that is not equal to zero, the form of the voltage across the load branch is changed such that the amplitude of the fundamental harmonic term of this voltage (the term of frequency f) decreases. As a result, also the power consumed by the load branch and the power consumed by the lamp decrease. The amplitude of the fundamental harmonic term of the voltage across the load branch decreases further as the second and the fifth time interval last longer. As a result, also the power consumed by the lamp decreases. The lowest power consumption by the lamp can be set by making the duration of both the second time interval and the fifth time interval equal to ⅙T, where T is the duration of a period of the control signal. It has been found that a circuit device in accordance with the invention enables the power consumed by the lamp to be adjusted in a comparatively large range without instabilities developing in the lamp.
Satisfactory results have been achieved with embodiments of a circuit device in accordance with the invention, wherein the duration of the second time interval is equal to the duration of the fifth time interval. The second and the fifth time interval can be made adjustable in a range from zero to ⅙T, as described hereinabove, where T is the duration of a period of the control signal. However, it is alternatively possible to make the second and the fifth time interval adjustable in a range from ⅙T to xc2xdT. In the latter case, the power consumed by the lamp is maximal if the second and the fifth time interval both have a duration equal to xc2xdT.
In a first preferred embodiment of a circuit device in accordance with the invention, xcex94t1/xcex94t3=1 and xcex94t4/xcex94t6=1 for each adjustable value of xcex94t2 and xcex94t5, where xcex94t1-xcex94t6 are the durations of, respectively, the first to the sixth time interval. As the second and the fifth time interval are in the middle of, respectively, the first half period and the second half period of the control signal, the durations of the second and the fifth time interval can be set in a large range.
In a second preferred embodiment of a circuit device in accordance with the invention, the dimming circuit is additionally provided with a circuit part FT for setting the point in time at which the second time interval begins within each first half period of the control signal, and for setting the point in time at which the fifth time interval begins within each second half period of the control signal. It has been found that, at predetermined durations of the second time interval and the fifth time interval, the power consumption by the lamp depends to a small degree on the points in time at which these time intervals begin in successive half periods. The circuit part FT thus enables the power consumption by the lamp to be very accurately adjusted.